I. Field of the Invention
The present invention pertains generally to the field of wireless communications, and more specifically to predistortion techniques for high power amplifiers.
II. Background
The field of wireless communications has many applications including, e.g., cordless telephones, paging, wireless local loops, and satellite communication systems. A particularly important application is cellular telephone systems for mobile subscribers. (As used herein, the term xe2x80x9ccellularxe2x80x9d systems encompasses both cellular and PCS frequencies.) Various over-the-air interfaces have been developed for such cellular telephone systems including, e.g., frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA). In connection therewith, various domestic and international standards have been established including, e.g., Advanced Mobile Phone Service (AMPS), Global System for Mobile (GSM), and Interim Standard 95 (IS-95). In particular, IS-95 and its derivatives, IS-95A, IS-95B, ANSI J-STD-008, etc. (often referred to collectively herein as IS-95), are promulgated by the Telecommunication Industry Association (TIA) and other well known standards bodies.
Cellular telephone systems configured in accordance with the use of the IS-95 standard employ CDMA signal processing techniques to provide highly efficient and robust cellular telephone service. An exemplary cellular telephone system configured substantially in accordance with the use of the IS-95 standard is described in U.S. Pat. No. 5,103,459, which is assigned to the assignee of the present invention and fully incorporated herein by reference. The aforesaid patent illustrates transmit, or forward-ink, signal processing in a CDMA base station. Exemplary receive, or reverse-link, signal processing in a CDMA base station is described in U.S. application Ser. No. 08/987,172, filed Dec. 9, 1997, entitled MULTICHANNEL DEMODULATOR, which is assigned to the assignee of the present invention and fully incorporated herein by reference.
In CDMA systems, over-the-air power control is a vital issue. An exemplary method of power control in a CDMA system is described in U.S. Pat. No. 5,056,109, which is assigned to the assignee of the present invention and fully incorporated herein by reference.
A primary benefit of using a CDMA over-the-air interface is that communications are conducted over the same RF band. For example, each mobile subscriber unit (typically a cellular telephone) in a given cellular telephone system can communicate with the same base station by transmitting a reverse-link signal over the same 1.25 MHz of RF spectrum. Similarly, each base station in such a system can communicate with mobile units by transmitting a forward-link signal over another 1.25 MHz of RF spectrum. It is to be understood that while 1.25 MHz is a preferred CDMA channel bandwidth, the CDMA channel bandwidth need not be restricted to 1.25 MHz, and could instead be any number, such as, e.g., 5 MHz.
Transmitting signals over the same RF spectrum provides various benefits including, e.g., an increase in the frequency reuse of a cellular telephone system and the ability to conduct soft handoff between two or more base stations. Increased frequency reuse allows a greater number of calls to be conducted over a given amount of spectrum. Soft handoff is a robust method of transitioning a mobile unit from the coverage area of two or more base stations that involves simultaneously interfacing with two base stations. (In contrast, hard handoff involves terminating the interface with a first base station before establishing the interface with a second base station.) An exemplary method of performing soft handoff is described in U.S. Pat. No. 5,267,261, which is assigned to the assignee of the present invention and fully incorporated herein by reference.
In conventional cellular telephone systems, a public switched telephone network (PSTN) (typically a telephone company) and a mobile switching center (MSC) communicate with one or more base station controllers (BSCs) over standardized E1 and/or T1 telephone lines (hereinafter referred to as E1/T1 lines). The BSCs communicate with base station transceiver subsystems (BTSs) (also referred to as either base stations or cell sites), and with each other, over a backhaul comprising E1/T1 lines. The BTSs communicate with mobile units (i.e., cellular telephones) via RF signals sent over the air.
As described in U.S. Pat. No. 5,103,459, a forward-link CDMA communication signal is modulated at a BTS with Walsh code covering and I and Q spreading. The resultant digital complex baseband CDMA waveform is then converted to an analog signal, upconverted to a carrier frequency signal, amplified with a high power amplifier (HPA), and sent to an antenna for RF transmission to a mobile unit. Due to the nonlinearity inherent in the HPA, the HPA stage is typically characterized by out-of-band (also known as sideband) emissions at the output of the HPA. The CDMA waveform for each frequency assignment at the output of the antenna must meet various out-of-band emission requirements pursuant to specifications set by the FCC and other regulatory bodies. It is necessary, therefore, that out-of-band emissions be attenuated.
In conventional systems, the out-of-band emissions were attenuated as required using analog filters following the HPA. However, this required extremely precise tuning of the filter to achieve the necessary accuracy. For enhanced accuracy, it would be desirable to provide a method of digitally attenuating out-of-band emissions. To decrease the number of transistors required in the HPA architecture and allow for the production of more efficient HPAs, predistortion may be applied at the input of the HPA. From a design standpoint, it is highly desirable that the HPA be able to work over a large range of input powers. However, a problem in designing a working HPA over such a large range of input powers is that the HPA will have a nonlinear gain and phase response at different input powers. Such nonlinearities result in undesired out-of-band emissions at the output of the HPA. Adjustments to the bias currents that increase the efficiency of the HPA further increase such nonlinear effects. Moreover, it would further be advantageous to provide a predistortion method that accounts for changes over time due to temperature variations and component aging. Thus, there is a need for a digital, adaptive predistortion technique that minimizes out-of-band emissions introduced by an HPA.
The present invention is directed to a digital, adaptive predistortion technique that minimizes out-of-band emissions introduced by an HPA. Accordingly, in one aspect of the invention, a method of reducing out-of-band emissions from an amplifier advantageously includes the steps of multiplying a digital input signal to the amplifier with a set of digital predistortion parameters, measuring the output power level of the out-of-band emissions from the amplifier, and adjusting the set of parameters based upon the measured output power level of the out-of-band emissions from the amplifier. In another aspect of the invention, a device for reducing out-of-band emissions from an amplifier advantageously includes complex multiplication logic for multiplying digital predistortion parameters with a digital input signal, an amplifier coupled to the complex multiplication logic, and a processor logically coupled to the amplifier for digitizing an output signal from the amplifier, processing the digitized output signal, and modifying the digital predistortion parameters based upon the output signal.